Paste- or grease-form compositions containing a silicone oil as a main component have so far been used in many industrial fields.
Particularly in medical and cosmetic industries, application of low viscosity silicone oils have been widely examined in recent years. This is because low viscosity silicone oils are not only have low toxicity and low skin stimulation, but also light in extension and excellent in refreshment feeling at the time of use.
In using low viscosity silicone oils, however, it is necessary to increase a compounding amount of a thickener (a consistency increasing agent). The use of a large amount of conventional thickener, including inorganic materials such as silica fine powder, kaolin, talc, sericite, bentonite, etc., and organic materials such as lithium soap, aluminum soap, etc. (reference: Plastic Zairyo Koza [9] Keiso Jushi, page 81, Nikkan Kogyo Shinbunsha), has the defects of being responsible for the difficulty in preparing a smooth and homogeneous composition, and making the prepared composition tend to separate and to discharge the silicone oil, that is, be deficient in storage stability.
Under these circumstances, relatively high viscosity silicone oils with a viscosity of 100 centistokes or above at 25.degree. C. have so far been used as a base oil in most cases.
However, paste- or grease-form silicone compositions using the high viscosity silicone oils described above as base oils are heavy in extension, and to make the matter worse, they leave a sticky feeling to skin after use. Therefore, cream-, stick-, ointment- and cake-form compositions utilizing such silicone compositions as described above naturally have their limitations in qualities.
On the other hand, conventional organosilica sols have generally been used as a thickener free from secondary aggregation. Those organosilica sols are usually prepared by converting colloidal silica produced from water glass into alcohol solutions through azeotropic distillation. Consequently, at the surfaces of individual silica particles are present the alkoxy groups produced by the reaction of silanol groups with the alcohol at a coverage of 1 to 3 micromol/m.sup.2 and unreacted silanol groups at a high coverage of 5 to 10 micromol/m.sup.2.
Therefore, these silica particles are poor in affinity for a silicone oil, and when such an organosilica sol as described above is compounded with a silicone oil, recombination reaction between silanol groups remaining unreacted at the surfaces of silica particles takes place at the time of mixing or removal of the solvent by distillation to cause aggregation of silica particles, and silica particles then separate as a precipitate from the silicone oil.
As described above, it was difficult to obtain compositions excellent in homogeneity even when a production process of compounding the conventional organosilica sol with a silicone oil was adopted.